1. Field of the Invention
The present embodiments relate to a fuel cell system for refilling fuel through a cartridge exchange, and in particular to a fuel cell system having a cartridge stored with liquid fuel and a method of computing the fuel level remaining in the cartridge.
2. Description of the Related Art
Generally, a fuel cell is a power generation system for directly converting chemical energy into electric energy by means of an electrochemical reaction of hydrogen and oxygen. As the hydrogen, pure hydrogen may be directly supplied to the fuel cell, or hydrogen obtained by reforming materials such as methanol, ethanol, natural gas, etc., may be supplied thereto. As the oxygen, pure oxygen may be directly supplied to the fuel cell system, or oxygen contained in general air using an air pump may be supplied thereto.
The fuel cell may be sorted into a polymer electrolyte membrane fuel cell and a direct methanol fuel cell operating at temperature of 100° C. or less, a phosphoric acid fuel cell operating at about 150 to 200° C., a molten carbonate fuel cell operating at a temperature of 600 to 700° C., and a solid oxide fuel cell operating at a temperature of 1000° C. or more. These respective fuel cells are basically the same in view of a principle of generating electricity, but use different sorts of fuels, catalysts, electrolytes, etc.
Among others, the direct methanol fuel cell (DMFC) directly uses fuel that is a mixture of high concentration methanol of liquid phase and water instead of using hydrogen as fuel. The direct methanol fuel cell is lower in output density than a fuel cell using hydrogen as direct fuel. However, the direct methanol fuel cell has advantages in that it has a high energy density per volume of methanol used as fuel, easily stores the fuel, and is adapted to a situation that the fuel cell is required to operate at a low output and for a long time. Also, the direct methanol fuel cell can be more compactly constituted because an additional device; such as a reformer for reforming fuel to generate hydrogen, etc., is not needed.
Also, the direct methanol fuel cell (DMFC) includes a membrane electrode assembly (MEA) including a polymer electrolyte membrane, and an anode electrode and a cathode electrode contacted to both sides of the polymer electrolyte membrane. As the polymer electrode membrane, fluoro polymer is used. However, since the methanol is excessively rapidly permeated into the fluoro polymer membrane, a crossover phenomenon transmitting non-reactive methanol to the polymer electrolyte membrane occurs when the direct methanol fuel cell uses high concentration methanol as fuel. Accordingly, in order to lower the concentration of methanol, the mixing fuel mixing methanol with water must be supplied to the fuel cell system.
In the case of the fuel cell as described above, in particular a portable DMFC, it is exemplary to use a fuel tank using a cartridge filled with fuel therein rather than to directly inject fluid fuel to the fuel tank in the fuel cell, considering the convenience of a user.
However, when the fuel of the fuel cartridge is exhausted, air is supplied to the MEA of the fuel cell, thereby bring out the fatal damage to the fuel cell system. Therefore, an apparatus for informing a user of the fuel exhaustion in the fuel tank or stopping the fuel cell system by sensing it beforehand is needed. However, since the cartridge type fuel cell system is a compact system, adding the apparatus for sensing the fuel exhaustion beforehand to the cartridge imposes a heavy burden on the fuel cell system in view of volume and cost.
Also, in the fuel cell system with a structure for preventing the injection of air to the MEA of the fuel cell, abruptly stopping electronic equipment due to the fuel exhaustion in the fuel cell system during the use of electronic equipment causes a user great inconvenience and a risk of data loss.